Diccionario de Unidades de Medida

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A Dictionary of Units

This provides a summary of most of the units of measurement to be found in use aroundthe world today (and a few of historical interest), together with the appropriateconversion factors needed to change them into a 'standard' unit of the S I.

The units may be found either by looking under the category in which they are used [such as length,mass, density, energy etc.], or else by picking one unit from an alphabetically ordered list of units.There are NO units of currency. There is an outline of the S I; a list of its basic defining standards andalso some of its derived units; then another list of all the S I prefixes and some notes on conventions ofusage. There is a short historical note on measures generally; descriptions of the Metric system, the UK (Imperial) system with a statement on the implementation of'metrication' in the U K, and the U Ssystem. Finally there is a list of other sources concerned with the topic of measures and units(including other Web sites) and also some notes about this document.

A Summary of the conversion factors most often required is available.

Summary table of conversion factors most of ten required

x means 'multiply by' . . . / means 'divide by' . . . # means it is an exact valueAll other values given to an appropriate degree of accuracy.

To change . . into . . do this . . To change . . into . . do this . .

acres hectares x 0.4047 kilograms ounces x 35.3

acres sq. kilometres / 247 kilograms pounds x 2.2046

acres sq. metres x 4047 kilograms tonnes / 1000 #

acres sq. miles / 640 # kilograms tons (UK/long) / 1016

barrels (oil) cu.metres / 6.29 kilograms tons (US/short) / 907

barrels (oil) gallons (UK) x 34.97 kilometres metres x 1000 #

barrels (oil) gallons (US) x 42 # kilometres miles x 0.6214

barrels (oil) litres x 159 litres cu.inches x 61.02

centimetres feet / 30.48 # litres gallons (UK) x 0.2200

centimetres inches / 2.54 # litres gallons (US) x 0.2642

centimetres metres / 100 # litres pints (UK) x 1.760

centimetres millimetres x 10 # litres pints (US liquid) x 2.113

cubic cm cubic inches x 0.06102 metres yards / 0.9144 #

cubic cm litres / 1000 # metres centimetres x 100 #

cubic cm millilitres x 1 # miles kilometres x 1.609

cubic feet cubic inches x 1728 # millimetres inches / 25.4 #

cubic feet cubic metres x 0.0283 ounces grams x 28.35

cubic feet cubic yards / 27 # pints (UK) litres x 0.5683

cubic feet gallons (UK) x 6.229 pints (UK) pints (US liquid) x 1.201cubic feet gallons (US) x 7.481 pints (US liquid) litres x 0.4732

cubic feet litres x 28.32 pints (US liquid) pints (UK) x 0.8327

cubic inches cubic cm x 16.39 pounds kilograms x 0.4536

cubic inches litres x 0.01639 pounds ounces x 16 #

cubic metres cubic feet x 35.31

______________________________________ ____________________________________

To change . . into . . do this . . To change . . into . . do this . .

square cm sq. inches x 0.1550

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feet centimetres x 30.48 # square feet sq. inches x 144 #

feet metres x 0.3048 # square feet sq. metres x 0.0929

feet yards / 3 # square inches square cm x 6.4516 #

fl.ounces (UK) fl.ounces (US) x 0.961 square inches square feet / 144 #

fl.ounces (UK) millilitres x 28.41 square km acres x 247

fl.ounces (US) fl.ounces (UK) x 1.041 square km hectares x 100 #

fl.ounces (US) millilitres x 29.57 square km square miles x 0.3861

gallons pints x 8 # square metres acres / 4047

gallons (UK) cubic feet x 0.1605 square metres hectares / 10 000 #

gallons (UK) gallons (US) x 1.2009 square metres square feet 10.76

gallons (UK) litres x 4.54609 # square metres square yards x 1.196

gallons (US) cubic feet x 0.1337 square miles acres x 640 #

gallons (US) gallons (UK) x 0.8327 square miles hectares x 259

gallons (US) litres x 3.785 square miles square km x 2.590

grams kilograms / 1000 # square yards square metres 1.196

grams ounces / 28.35 tonnes kilograms x 1000 #

hectares acres x 2.471 tonnes tons (UK/long) x 0.9842

hectares square km / 100 # tonnes tons (US/short) x 1.1023hectares square metres x 10000 # tons (UK/long) kilograms x 1016

hectares square miles / 259 tons (UK/long) tonnes x 1.016

hectares square yards x 11 960 tons (US/short) kilograms x 907.2

inches centimetres x 2.54 # tons (US/short) tonnes x 0.9072

inches feet / 12 # yards metres x 0.9144 #

The Systeme International [S I]

Le Systeme international d'Unites officially came into being in October 1960 and has been adopted by nearly all countries,though the amount of actual usage varies considerably.

It is based upon 7 principal units, 1 in each of 7 different categories -

Cat egor y Name Abbr ev i a t i on

Lengt h met r e mMass ki l ogr am kgTi me second sEl ect r i c cur r ent amper e ATemperat ure kel vi n KAmount of subst ance mol e molLumi nous i nt ensi t y candel a cd

Definitions of these basic units are given. Each of these units may take a prefix. From these basic units many other units arederived and named.

Return to the top of this document

Definitions of the Seven Basic S I Units

metre [m]The metre is the basic unit of length. It is the distance light travels, in a vacuum, in 1/299792458th of a second.

kilogram [kg]

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The kilogram is the basic unit of mass. It is the mass of an international prototype in the form of a platinum-iridiumcylinder kept at Sevres in France.It is now the only basic unit still defined in terms of a material object, and also theonly one with a prefix[kilo] already in place.

second [s]The second is the basic unit of time. It is the length of time taken for 9192631770 periods of vibration of thecaesium-133 atom to occur.

ampere [A]The ampere is the basic unit of electric current. It is that current which produces a specified force between twoparallel wires which are 1 metre apart in a vacuum.It is named after the French physicist Andre Ampere (1775-1836).

kelvin [K]The kelvin is the basic unit of temperature. It is 1/273.16th of the thermodynamic temperature of the triple point ofwater. It is named after the Scottish mathematician and physicist William Thomson 1st Lord Kelvin (1824-1907).

mole [mol]The mole is the basic unit of substance. It is the amount of substance that contains as many elementary units as thereare atoms in 0.012 kg of carbon-12.

candela [cd]The candela is the basic unit of luminous intensity. It is the intensity of a source of light of a specified frequency,which gives a specified amount of power in a given direction.


Derived Units of the S IFrom the 7 basic units of the SI many other units are derived for a variety of purposes. Only some of them are explained here.The units printed in bold are either basic units or else, in some cases, are themselves derived.farad [F]

The farad is the SI unit of the capacitance of an electrical system, that is, its capacity to store electricity. It is a ratherlarge unit as defined and is more often used as a microfarad. It is named after the English chemist and physicistMichael Faraday (1791-1867).

hertz [Hz]The hertz is the SI unit of the frequency of a periodic phenomenon. One hertz indicates that 1 cycle of thephenomenon occurs every second. For most work much higher frequencies are needed such as the kiloherz [kHz]and megaherz [MHz].It is named after the German physicist Heinrich Rudolph Herz (1857-94).

joule [J]The joule is the SI unit of work or energy. One joule is the amount of work done when an applied force of 1 newton

moves through a distance of 1 metre in the direction of the force.It is named after the English physicist JamesPrescott Joule (1818-89).

newton [N]The newton is the SI unit of force. One newton is the force required to give a mass of 1 kilogram an acceleration of1 metre per second per second.It is named after the English mathematician and physicist Sir Isaac Newton(1642-1727).

ohm [*]The ohm is the SI unit of resistance of an electrical conductor. Its symbol, shown here as [*] is the Greek letterknown as 'omega'.It is named after the German physicist Georg Simon Ohm (1789-1854).

pascal [Pa]The pascal is the SI unit of pressure. One pascal is the pressure generated by a force of 1 newton acting on an areaof 1 square metre. It is a rather small unit as defined and is more often used as a kilopascal [kPa].It is named afterthe French mathematician, physicist and philosopher Blaise Pascal (1623-62).

volt [V]

The volt is the SI unit of electric potential. One volt is the difference of potential between two points of an electicalconductor when a current of 1 ampere flowing between those points dissipates a power of 1 watt.It is named afterthe Italian physicist Count Alessandro Giuseppe Anastasio Volta (1745-1827).

watt [W]The watt is used to measure power or the rate of doing work. One watt is a power of 1 joule per second.It isnamed after the Scottish engineer James Watt (1736-1819).

Note that prefixes may be used in conjunction with any of the above units.





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The Prefixes of the S I

The S I allows the sizes of units to be made bigger or smaller by the use of appropriate prefixes. For example, the electricalunit of a watt is not a big unit even in terms of ordinary household use, so it is generally used in terms of 1000 watts at atime. The prefix for 1000 is kilo so we use kilowatts[kW] as our unit of measurement. For makers of electricity, or biggerusers such as industry, it is common to use megawatts[MW] or even gigawatts[GW]. The full range of prefixes with their[symbols or abbreviations] and their multiplying factors which are also given in other forms is

yot t a [Y] 1 000 000 000 000 000 000 000 000 = 10 24zett a [ Z] 1 000 000 000 000 000 000 000 = 10 21exa [ E] 1 000 000 000 000 000 000 = 10 18pet a [ P] 1 000 000 000 000 000 = 10 15t er a [ T] 1 000 000 000 000 = 10 12gi ga [ G] 1 000 000 000 ( a t hous and mi l l i ons = a bi l l i on) mega [ M] 1 000 000 ( a mi l l i on) ki l o [ k] 1 000 ( a t h ou s an d) hecto [ h] 100deca [ da] 10

1deci [ d] 0. 1cent i [ c] 0. 01mi l l i [ m] 0. 001 ( a t h ou s an dt h )

mi cr o [ ] 0. 000 001 ( a mi l l i on t h) nano [ n] 0. 000 000 001 ( a t hous and mi l l i ont h) pi co [ p] 0. 000 000 000 001 = 10 - 12f emt o [ f ] 0. 000 000 000 000 001 = 10 - 15at t o [ a] 0. 000 000 000 000 000 001 = 10 - 18zepto [z ] 0. 000 000 000 000 000 000 001 = 10 - 21yoct o [y] 0. 000 000 000 000 000 000 000 001 = 10 - 24

[] the symbol used for micro is the Greek letter known as 'mu'Nearly all of the S I prefixes are multiples or sub-multiples of 1000. However, these are inconvenient for many purposes andso hecto, deca, deci, and centi are also used.deca also appears as deka [da] or [dk] in the USA and Contintental Europe. So much for standards!


Conventions of Usage in the S I

There are various rules laid down for the use of the SI and its units as well as some observations to be made that will help inits correct use.

Any unit may take only ONE prefix. For example 'millimillimetre' is incorrect and should be written as 'micrometre'.Most prefixes which make a unit bigger are written in capital letters (M G T etc.), but when they make a unit smallerthen lower case (m n p etc.) is used. Exceptions to this are the kilo [k] to avoid any possible confusion with kelvin[K]; hecto [h]; and deca [da] or [dk]A unit which is named after a person is written all in lower case (newton, volt, pascal etc.) when named in full, butstarting with a capital letter (N V Pa etc.) when abbreviated. An exception to this rule is the litre which, if written as alower case 'l' could be mistaken for a '1' (one) and so a capital 'L' is allowed as an alternative. It is intended that asingle letter will be decided upon some time in the future when it becomes clear which letter is being favoured most inuse.Units written in abbreviated form are NEVER pluralised. So 'm' could always be either 'metre' or 'metres'. 'ms'could represent 'metre second' (whatever that is) or, more correctly, 'millisecond'.An abbreviation (such as J N g Pa etc.) is NEVER followed by a full-stop unless it is the end of a sentence.To make numbers easier to read they may be divided into groups of 3 separated by spaces (or half-spaces) but NOTcommas.The SI preferred way of showing a decimal fraction is to use a comma (123,456) to separate the whole number fromits fractional part. The practice of using a point, as is common in English-speaking countries, is acceptable providingonly that the point is placed ON the line of the bottom edge of the numbers (123.456).It will be noted that many units are eponymous, that is they are named after persons. This is always someone whowas prominent in the early work done within the field in which the unit is used.




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A Brief History of Measurement

One of the earliest types of measurement concerned that of length. These measurements were usually based on parts of thebody. A well documented example (the first) is the Egyptian cubit which was derived from the length of the arm from theelbow to the outstretched finger tips. By 2500 BC this had been standardised in a royal master cubit made of black marble

(about 52 cm). This cubit was divided into 28 digits (roughly a finger width) which could be further divided into fractionalparts, the smallest of these being only just over a millimetre.

In England units of measurement were not properly standardised until the 13th century, though variations (and abuses)continued until long after that. For example, there were three different gallons (ale, wine and corn) up until 1824 when thegallon was standardised.

In the U S A the system of weights and measured first adopted was that of the English, though a few differences came inwhen decisions were made at the time of standardisation in 1836. For instance, the wine-gallon of 231 cubic inches was usedinstead of the English one (as defined in 1824) of about 277 cubic inches. The U S A also took as their standard of drymeasure the old Winchester bushel of 2150.42 cubic inches, which gave a dry gallon of nearly 269 cubic inches.

Even as late as the middle of the 20th century there were some differences in UK and US measures which were nominally the

same. The UK inch measured 2.53998 cm while the US inch was 2.540005 cm. Both were standardised at 2.54 cm in July1959, though the U S continued to use 'their' value for several years in land surveying work - this too is slowly beingmetricated.

In France the metric system officially started in June 1799 with the declared intent of being 'For all people, for all time'. Theunit of length was the metre which was defined as being one ten-millionth part of a quarter of the earth's circumference. Theproduction of this standard required a very careful survey to be done which took several years. However, as more accurateinstruments became available so the 'exactness' of the standard was called into question. Later efforts were directed at findingsome absolute standard based on an observable physical phenomenon. Over two centuries this developed into the S I. Somaybe their original slogan was more correct than anyone could have foreseen then.


Metric System of Measurements

Lengt h Ar ea10 mi l l i met r es = 1 cent i met re 100 sq. mm = 1 sq. cm10 cent i met res = 1 deci meter 10 000 sq. cm = 1 sq. met r e10 deci met r es = 1 met r e 100 sq. met r es = 1 ar e10 met r es = 1 decamet r e 100 ar es = 1 hect ar e10 decamet res = 1 hectomet re 10 000 sq. met res = 1 hectare10 hectometr es = 1 ki l ometr e 100 hectares = 1 sq. ki l omet r e

1000 metr es = 1 ki l ometr e 1 000 000 sq. metr es = 1 sq. ki l ometr e

Vol ume Capaci t y1000 cu. mm = 1 cu. cm 10 mi l l i l i t r es = 1 cent i l i t r e1000 cu. cm = 1 cu. deci met r e 10 cent i l i t r ee = 1 deci l i t r e1000 cu. dm = 1 cu. met r e 10 deci l i t r es = 1 l i t r e

1 mi l l i on cu. cm= 1 cu. met re 1000 l i t r es = 1 cu. met re

Mas s1000 grams = 1 ki l ogram1000 ki l ograms = 1 tonne

The distinction between 'Volume' and 'Capacity' is artificial and kept here only for historic reasons.A millitre is a cubic centimetre and a cubic decimetre is a litre. But see under 'Volume' for problems with the litre.




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The U K (Imperial) System of Measurements

Lengt h Ar ea12 i nches = 1 f oot 144 sq. i nches = 1 squar e f oot3 f eet = 1 yar d 9 sq. f eet = 1 squar e yar d

22 yar ds = 1 chai n 4840 sq. yar ds = 1 acr e10 chai ns = 1 f ur l ong 640 acr es = 1 squar e mi l e8 f ur l ongs = 1 mi l e

5280 f eet = 1 mi l e1760 yards = 1 mi l e Capaci t y

20 f l ui d ounces = 1 pi ntVol ume 4 gi l l s = 1 pi nt

1728 cu. i nches = 1 cubi c f oot 2 pi nt s = 1 quar t27 cu. f eet = 1 cubi c yar d 4 quar t s = 1 gal l on ( 8 pi nt s )

Mass ( Avoi r dupoi s) 437. 5 gr ai ns = 1 ounce Tr oy Wei ght s16 ounces = 1 pound ( 7000 gr ai ns) 24 gr ai ns = 1 pennywei ght14 pounds = 1 st one 20 pennywei ght s = 1 ounce ( 480 gr ai ns)

8 st ones = 1 hundr edwei ght [ cwt ] 12 ounces = 1 pound ( 5760 gr ai ns)20 cwt = 1 t on ( 2240 pounds)

Apothecari es' Measures Apothecari es' Wei ghts20 mi ni ms = 1 f l . s cr upl e 20 gr ai ns = 1 scr upl e3 f l . scr upl es = 1 f l . dr achm 3 scr upl es = 1 dr achm8 f l . dr achms = 1 f l . ounce 8 dr achms = 1 ounce ( 480 gr ai ns)

20 f l . ounces = 1 pi nt 12 ounces = 1 pound ( 5760 gr ai ns)

The old Imperial (now UK) system was originally defined by three standard measures - the yard, the pound and the gallonwhich were held in London. They are now defined by reference to the S I measures of the metre, the kilogram and the litre.These equivalent measures are exact.

1 yard = 0.9144 metres - same as US

1 pound = 0.453 592 37 kilograms - same as US1 gallon = 4.546 09 litresNote particularly that the UK gallon is a different size to the US gallon so that NO liquid measures of the same name are thesame size in the UK and US systems.Also that the ton(UK) is 2240 pounds while a ton(US) is 2000 pounds. These are also referred to as a long ton and short tonrespectively.


Metrication in the U K

There have been three major Weights and Measures Acts in recent times (1963, 1976 and 1985) all gradually abolishing

various units, as well re-defining the standards. All the Apothecaries' measures are gone, and of the Troy measures, only theounce remains. Currently legislation has decreed that -

From the 1st October 1995, for economic, public health, public safety and administrative purposes, only metric units areallowed EXCEPT that -

pounds and ounces for weighing of goods sold from bulkpints and fluid ounces for beer, cider, waters, lemonades and fruit juices in RETURNABLE containerstherms for gas supplyfathoms for marine navigation

may be used until 31st December 1999.




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The following may continue to be used WITHOUT time limit -

miles, yards, feet and inches for road traffic signs and related measurements of speed and distancepints for dispensing draught beer and cider, and for milk in RETURNABLE containersacres for land registration purposestroy ounces for transactions in precious metals.

Sports are exempt from all of this, but most of them have (voluntarily) changed their relevant regulations into statements of

equivalent metric measures.

That is how the legislation is framed. In common usage the 'old' units are still very apparent.


The U S System of Measurements

Most of the US system of measurements is the same as that for the UK. The biggest differences to be noted are in Capacitywhich has both liquid and dry measures as well as being based on a different standard - the US liquid gallon is smaller thanthe UK gallon. There is also a measurement known at the US survey foot. It is gradually being phased out as the maps andland plans are re-drawn under metrication. (The changeover is being made by putting 39.37 US survey feet = 12 metres)

Lengt h Ar ea12 i nches = 1 f oot 144 sq. i nches = 1 squar e f oot3 f eet = 1 yar d 9 sq. f eet = 1 squar e yar d

220 yar ds = 1 f ur l ong 4840 sq. yar ds = 1 acr e8 f ur l ongs = 1 mi l e 640 acr es = 1 square mi l e

5280 f eet = 1 mi l e 1 sq. mi l e = 1 sect i on1760 yar ds = 1 mi l e 36 sect i ons = 1 t ownshi p

Vol ume1728 cu. i nches = 1 cubi c f oot

27 cu. f eet = 1 cubi c yar d

Capaci t y ( Dr y) Capaci t y ( Li qui d)

16 f l ui d ounces = 1 pi nt2 pi nt s = 1 quar t 4 gi l l s = 1 pi nt8 quar t s = 1 peck 2 pi nt s = 1 quar t4 pecks = 1 bushel 4 quart s = 1 gal l on ( 8 pi nt s)

Mas s 437. 5 gr ai ns = 1 ounce Tr oy Wei ght s16 ounces = 1 pound ( 7000 gr ai ns) 24 gr ai ns = 1 pennywei ght14 pounds = 1 st one 20 pennywei ght s = 1 ounce ( 480 gr ai ns)

100 pounds = 1 hundr edwei ght [ cwt ] 12 ounces = 1 pound ( 5760 gr ai ns)20 cwt = 1 t on ( 2000 pounds)

Apothecar i es' Measures Apothecar i es' Wei ght s60 mi ni ms = 1 f l . dr am 20 gr ai ns = 1 scr upl e

8 f l . dr ams = 1 f l . ounce 3 scr upl es = 1 dr am16 f l . ounces = 1 pi nt 8 dr ams = 1 ounce ( 480 gr ai ns)12 ounces = 1 pound ( 5760 grai ns)

As with the UK system these measures were originally defined by physical standard measures - the yard, the pound, thegallon and the bushel.They are now all defined by reference to the S I measures of the metre, the kilogram and the litre. Theseequivalent measures are exact.

1 yard = 0.9144 metres - same as UK1 pound = 0.453 592 37 kilograms - same as UK1 gallon (liquid) = 3.785 411 784 litres1 bushel = 35.239 070 166 88 litres




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Note particularly that the US gallon is a different size to the UK gallon so that NO liquid measures of the same name are thesame size in the US and UK systems.Also that the ton(US) is 2000 pounds while a ton(UK) is 2240 pounds. These are also referred to as a short ton and long tonrespectively.Note than in matters concerned with land measurements, for the most accurate work, it is necessary to establish whether theUS survey measures are being used or not.


Categories of Units

lengthareavolume or capacitymasstemperaturedensity, areadensity, linedensity, volumeenergyforcefuel consumptionline densitymass per unit lengthmass per unit areamass per unit volumepowerpressurespeedspread rate (by mass)spread rate (by volume)

stresstorque


List of Units

The units are listed in alphabetical order but scanning can be speeded up by selectingthe initial letter of the unit from these individual letters or groups

A - B - C - D - E - F - G - H - IJ - K - L - M

N - O - PQ - R - S - T - UVW - XYZ

A

acresangstroms




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aresastronomical unitsatmospheres

B

barleycornsbarrels (oil)barsBritish thermal unitsBtu/hour etc.bushels

C

caloriescalories per hour etc.carats, metric

Celsiuscentigradecentigrade heat unitscentilitrescentimetrescentimetres of mercury or watercentimetres per minute etc.chains (surveyors')circular inchescubic (+ any units)cubic measures per areacubits

D

decilitresdenierdrexdynes

E

ells (UK)ems (pica)

ergs (energy)ergs (torque)

F

Fahrenheitfathomsfeetfeet of water




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feet per hour etc.fluid ouncesfoot pounds-forcefoot pounds-force per minute etc.foot poundalsfurlongs

G

gallonsgallons per areagigajoulesgigawattsgrainsgrains per gallongramsgram-force centimetresgrams per areagrams per cmgrams per (any volume)

H

handshectareshideshorsepowerhorsepower hourshundredweights

IJinchesinches of mercury or waterinches of rain (by mass)inches of rain (by volume)inches per minute etc.joulesjoules per hour etc.

K

Kelvinkilocalorieskilocalories per hour etc.kilograms-forcekilogram-force metres (energy)kilogram-force metres (torque)kilogram-force metres per hour etc.kilogram-force per areakilogramskilograms per areakilograms per metrekilograms per volume




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kilojouleskilojoules per hour etc.kilometreskilometres per hour etc.kilometres per litrekilonewton per square metrekilonewtonskilopascalskilowatts

kilowatt hourskips (weight)kips (force)kips per square inchknots

L

leagueslight yearslinks (surveyors')litreslitres per area

M

Mach numbermegajoulesmeganewtonsmeganewtons per square metremegawattsmetresmetres of water

metres per second etc.microns (=micrometres)milesmiles per gallonmiles per hour etc.millibarsmilligrams per cmmilligrams per (any volume)millilitresmillimetres of mercury or watermillimetres of rain (by mass)millimetres of rain (by volume)

N

newton metres (energy)newton metres (torque)newtons (per area)newtons (force)newtons (weight)

O




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ouncesounces per inchounces per areaounces per volume

PQ

parsecspascalsperch (=rods or poles)picaspintspoints (printers')poundalspoundals per square footpoundspounds per areapounds per footpounds per volumepounds-force

pound-force inchespounds-force per areaquarts

R

RankineReaumurroods

S

slugs (or g-pounds)stonessquare (+ any units)squares (of timber)sthenes

T

tex

thermstonneston-force metrestonnes-forcetonnes-force per areatonnes per hectaretonnes per kmtonnes per volumeton-force feettonstons-force




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tons per acretons per miletons per volumetownshipstroy ounce

UVW

watt secondwatt hourswatts

XYZ

yardsyards per hour etc.


Length

The S I unit of length is the metre. To change any of these other units of length into their equivalent values inmetres use the operation and conversion factor given. Those marked with # are exact. Other values are given to anappropriate degree of accuracy. Where some uncertainty is indicated it means that a good idea of the size of the unitcan be given but that a better value would depend upon knowing the period and/or culture in which the unit wasbeing used.

angst r oms di vi de by 10 000 000 000 #ast r onomi cal uni t s x 149 598 550 000bar l eycor ns x 0. 008 467

cent i met r es x 0. 01 #chai ns ( s ur veyor s' ) x 20. 1168 #cubi t s x ( 0. 45 t o 0. 5)el l s ( UK) x 0. 875 ( b ut ma ny v ar i a t i o ns ) ems ( pi ca) x 0. 004 233 3f at homs x 1. 8288 #

f eet ( UK and US) x 0. 3048 #f eet ( US sur vey) x 0. 304 800 609 6f ur l ongs x 201. 168 #hands x 0. 106 #i nches x 0. 0254 #ki l omet r es x 1000 #l eagues x ( 4000 t o 5000)

l i ght year s x 9 460 500 000 000 000l i nks ( sur veyor s' ) x 0. 201 168 #

met r es [ m] 1

mi cr ons ( =mi cr omet r es) x 0. 000 001 #mi l es ( UK and US) x 1609. 344 #mi l es ( naut i cal ) x 1852 #par secs x 30 856 770 000 000 000per ch ( =r ods or pol es) x 5. 0292 #pi cas ( comput er ) x 0. 004 233 333




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pi cas ( pr i nt er s' ) x 0. 004 217 518poi nt s ( c omput er ) x 0. 000 352 777 8poi nt s ( pr i nt er s' ) x 0. 000 351 459 8yar ds x 0. 9144 #

Note than in matters concerned with land measurements, for the most accurate work, it is necessary to establishwhether the US survey measures are being used or not.


Area

The S I unit of area is the square metre. To change any of these other units of area into their equivalent valuesin square metres use the operation and conversion factor given. Those marked with # are exact. Other values aregiven to an appropriate degree of accuracy. Where some uncertainty is indicated it means that a good idea of the sizeof the unit can be given but that a better value would depend upon knowing the period and/or culture in which theunit was being used.

acr es x 4046. 856 422 4 #ar es x 100 #ci r cul ar i nches x 0. 000 506 707 479hect ar es x 10 000 #

hi des x 485 000 ( wi t h wi de v ar i at i ons ) r oods x 1011. 714 105 6 #squar e cent i met r es x 0. 000 1 #square f eet ( UK and US) x 0. 092 903 04 #square f eet ( US sur vey) x 0. 092 903 411 613squar e i nches x 0. 000 645 16 #squar e ki l omet r es x 1 000 000 #

squar e met r es 1

squar e mi l es x 2 589 988. 110 336 #squar e mi l l i met r es x 0. 000 001 #squares (of t i mber ) x 9. 290 304 #squar e rods ( or pol es) x 25. 292 852 64 #

squar e yar ds x 0. 836 127 36 #t ownshi ps x 93 239 571. 972

Note than in matters concerned with land measurements, for the most accurate work, it is necessary to establishwhether the US survey measures are being used or not.


Volume or Capacity

The S I unit of volume is the cubic metre. However, this seems to be much less used than the litre (1000 litres = 1cubic metre).To change any of these other units of volume into their equivalent values in litres use the operation

and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree ofaccuracy.The litre. There can be some ambiguity about the size of the litre. In 1901 it was defined by reference to a kilogramof pure water under certain particular conditions. (This was similar to the way the old UK gallon was set.) In 1964 itwas re-defined as a common usage term for a cubic decimetre. They differ very slightly and for really accurate work,to avoid any possible confusion, it is recommended that the litre is not used . It is used here as being a cubicdecimetre.

bar r el s ( oi l ) x 158. 987 294 928 #bushel s ( UK) x 36. 368 72 #bushel s ( US) x 35. 239 070 166 88 #cent i l i t r es x 0. 01 #




15/25

cubi c cent i met r es x 0. 001 #cubi c deci met r es 1cubi c decamet r es x 1 000 000 #cubi c f eet x 28. 316 846 592 #cubi c i nches x 0. 016 387 064 #

cubi c met r es x 1000 #cubi c mi l l i met r es x 0. 000 001 #cubi c yards x 764. 554 857 984 #

deci l i t r es x 0. 1 #f l ui d ounces ( UK) x 0. 028 413 062 5 #f l ui d ounces (US) x 0. 029 573 529 562 5 #gal l ons ( UK) x 4. 546 09 #gal l ons, dr y ( US) x 4. 404 883 770 86 #gal l ons , l i qui d (US) x 3. 785 411 784 #

l i t r es [ l or L] 1

l i t r es (1901 - 1964) x 1. 000 028mi l l i l i t r es x 0. 001 #pi nt s ( UK) x 0. 568 261 25 #pi nt s, dr y ( US) x 0. 550 610 471 357 5 #pi nt s , l i qui d ( US) x 0. 473 176 473 #

quar t s ( UK) x 1. 136 522 5 #quar t s , dr y ( US) x 1. 101 220 942 715 #quar t s , l i qui d (US) x 0. 946 352 946 #


Mass (or Weight)

The S I unit of mass is the kilogram. To change any of these other units of mass into their equivalent values inkilograms use the operation and conversion factor given. Those marked with # are exact. Other values are given toan appropriate degree of accuracy.

car at s, met r i c x 0. 000 2 #

gr ai ns x 0. 000 064 798 91 #gr ams x 0. 001 #hundr edwei ght s, l ong x 50. 802 345 44 #hundr edwei ght s, shor t x 45. 359 237 #

ki l ogr ams [ kg] 1

ounces, avoi rdupoi s x 0. 028 349 523 125 #ounces, t r oy x 0. 031 103 476 8 #pounds x 0. 453 592 37 #s lugs (or g-pounds ) x 14. 593 903st ones x 6. 350 293 18 #t ons ( UK or l ong) x 1016. 046 908 8 #t ons ( US or shor t ) x 907. 184 74 #

t onnes x 1000 #


Temperature

There have been five main temperature scales, each one being named after the person who invented it.G D FAHRENHEIT (1686-1736) a German physicist, in about 1714 proposed the first practical scale. He called thefreezing-point of water 32 degrees (so as to avoid negative temperatures) and the boiling-point 212 degrees.R A F de REAUMUR (1673-1757) A French entomologist, proposed a similar scale in 1730, but set the




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freezing-point at 0 degrees and the boiling-point at 80 degrees. This was used quite a bit but is now obsolete.Anders CELSIUS (1701-1744) a Swedish astronomer, proposed the 100-degree scale (from 0 to 100) in 1742. Thiswas widely adopted as the centigrade scale. But since grades and centigrades were also measures of angle, in 1948 itofficially became the Celsius scale. Also, the S I system of units gives preference to naming units after people wherepossible.William Thomson, 1st Lord KELVIN (1824-1907) a Scottish mathematician and physicist, worked with J P Joule -about 1862 - to produce an absolute scale of temperature based on laws of heat rather than the freezing/boiling-pointsof water. This work produced the idea of 'absolute zero', a temperature below which it was not possible to go. Itsvalue is -273.15 degrees on the Celsius scale.

William J M RANKINE (1820-1872) a Scottish engineer and scientist, promoted the Kelvin scale in its Fahrenheitform, when the equivalent value of absolute zero is -459.67 degrees Fahrenheit.Nowadays, while scientists use the KELVIN scale, the CELSIUS scale is the preferred scale in our everyday lives.However, the Fahrenheit scale is still widely used and there frequently is a need to be able to change from one to theother.To change temperature given in Fahrenheit (F) to Celsius (C)

St ar t wi t h ( F) ; subt r act 32; mul t i pl y by 5; di vi de by 9; t he answer i s ( C)

To change temperature given in Celsius (C) to Fahrenheit (F)

St ar t wi t h ( C) ; mul t i pl y by 9; di vi de by 5; add on 32; t he answer i s ( F)


Line density

Line density is a measure of mass per unit length. The S I compatible unit of line density is kilograms/metre. Amajor use of line density is in the textile industry to indicate the coarseness of a yarn or fibre. For that purpose the SIunit is rather large so the preferred unit there is the tex. (1 tex = 1 gram/kilometre) To change any of these other unitsof line density into their equivalent values in kilograms/metre use the operation and conversion factor given.Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

deni er di vi de by 9 000 000 #dr ex di vi de by 10 000 000 #gr ams/ cent i met r e di vi de by 10 #grams/ ki l omet r e ( t ex) di vi de by 1 000 000 #

gr ams/ met r e di vi de by 1000 #gr ams/ mi l l i met r e 1ki l ograms /ki l omet r e di vi de by 1000 #

ki l ogr ams/ met r e 1

mi l l i grams/ cent i met re di vi de by 10 000 #mi l l i grams/ mi l l i met re di vi de by 1000 #ounces/ i nch x 1. 116 125ounces/ f oot x 0. 093 01pounds/ i nch x 17. 858pounds/ f oot x 1. 488 164pounds/ yar d x 0. 496 055pounds/ mi l e x 0. 000 281 849

t ex di vi de by 1 000 000 #t ons( UK) / mi l e x 0. 631 342t ons( US) / mi l e x 0. 563 698t onnes/ ki l omet r e 1


Density

Density is the shortened term generally used in place of the more accurate description volumetric density.It is a




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measure of mass per unit volume. The S I compatible unit of density is kilograms/cubic metre. However, this arather large unit for most purposes (iron is over 7000, wood is about 600 and even cork is over 200). A much moreuseful size of unit is kilograms/litre (for which the previous values then become 7, 0.6 and 0.2 respectively). Thisunit also has the great advantage of being numerically unchanged for grams/cubic centimetre and tonnes/cubic metre(or megagrams/cubic metre). To change any of these other units of density into their equivalent values inkilograms/litre use the operation and conversion factor given. Those marked with # are exact. Other values aregiven to an appropriate degree of accuracy.

gr ai ns/ gal l on( UK) di vi de by 70 156gr ai ns/ gal l on( US) di vi de by 58 418

grams/ cubi c cent i met re 1gr ams/ l i t r e di vi de by 1000 #gr ams/ mi l l i l i t r e 1ki l ogr ams/ cubi c met r e di vi de by 1000 #megagr ams/ cubi c met r e 1mi l l i grams /mi l l i l i t r e di vi de by 1000 #mi l l i gr ams/ l i t r e di vi de by 1 000 000 #

ki l ograms/ l i t r e 1

ounces/ c ubi c i nch x 1. 729 994 044ounces/ gal l on( UK) x 0. 006 236 023ounces/ gal l on( US) x 0. 007 489 152pounds/ cubi c i nch x 27. 679 904

pounds/ c ubi c f oot x 0. 016 018 463pounds/ gal l on( UK) x 0. 099 776 373pounds/ gal l on( US) x 0. 119 826 427t onnes/ cubi c met re 1tons(UK) / cubi c yard x 1. 328 939 184tons(US) / cubi c yard x 1. 186 552 843


Energy or work

There is a lot of room for confusion in some of the units used here. The calorie can take 5 different values and,

while these do not vary by very much, for accurate work it is necessary to specify which calorie is being used.The 5 calories are known as the International Table calorie - cal(IT); the thermochemical calorie - cal(th); the meancalorie - cal(mean); the 15 degree C calorie - cal(15C); and the 20 degree C calorie - cal(20C).As a further complication, in working with food and expressing nutritional values, the unit of a Calorie (capital C) isoften used to represent 1000 calories, and again it is necessary to specify which calorie is being used for that.The British thermal unit (Btu) can also take different values and they are named in a similar way to the calorie,that is Btu (IT), (th), etc. Also note that the therm is 100 000 Btu so its exact size depends on which Btu is beingused.

The S I unit of energy or work is thejoule. To change any of these other units of energy or work into theirequivalent values in joules use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

Br i t i sh t her mal uni t s( I T) x 1055. 056Bt u ( t h) x 1054. 350

Bt u (mean) x 1055. 87cal or i es - cal ( I T) x 4. 1868 #

- cal ( t h) x 4. 184 #- cal ( mean) x 4. 190 02- cal ( 15C) x 4. 185 80- cal ( 20C) x 4. 181 90

Cal or i e ( f ood) x 4186 ( appr ox. )cent i grade heat uni t s x 1900. 4er gs di vi de by 10 000 000 #f oot pounds- f or ce x 1. 355 817f oot poundal s x 0. 042 140




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gi gaj oul es [ GJ ] x 1000 000 000 #hor sepower hours x 2 684 520 ( approx. )

j oul es [ J ] 1

ki l ocal or i es ( I T) x 4186. 8 #ki l ocal or i es ( th) x 4184 #ki l ogram- f orce met res x 9. 806 65 #ki l oj oul es [k J ] x 1000 #

ki l owatt hours [ kWh] x 3 600 000 #megaj oul es [ MJ ] x 1 000 000 #newt on met r es [ Nm] x 1 #t her ms x 105 500 000 ( appr ox. )wat t seconds [ Ws] 1wat t hour s [ Wh] x 3600 #


Force

The S I unit of force is the newton. To change any of these other units of force into their equivalent values in

newtons use the operation and conversion factor given. Those marked with # are exact. Other values are given toan appropriate degree of accuracy.dynes di vi de by 100 000 #ki l ogr ams f or ce x 9. 806 65 #ki l onewt ons [ kN] x 1000 #ki ps x 4448. 222meganewt ons [ MN] x 1 000 000 #

newt ons [ N] 1

pounds f or ce x 4. 448 222poundal s x 0. 138 255st henes ( =kN) x 1000t onnes f or ce x 9806. 65 #

t ons( UK) f or ce x 9964. 016t ons( US) f or ce x 8896. 443


Fuel Consumption

Fuel consumption of any means of transport (car, aeroplane, ship etc.) that uses fuel is a measure giving therelationship between the distance travelled for an amount of fuel used. The most common example is the car where itis usually expressed (in English-speaking countries) in miles per gallon.It could also be expressed in gallons per mile. However, for a car the latter method gives a rather small figure: 35miles per gallon is about 0.0286 gallons per mile. In that case it would be better to give a figure for 100 miles, so itwould be 2.86 gallons per 100 miles. That is the metric way of expressing fuel consumption - as litres per 100kilometres.From regular enquiries it appears that in real life people are using all sorts of ways of expressing their fuelconsumption, so this section (unlike all the others) tries to cover as many ways as possible. All the values are givento an accuracy of 4 significant figures.

To change i nt omi l es per gal l on ( UK) mi l es per gal l on ( US) mul t i pl y by 0. 833mi l es per gal l on ( UK) mi l es per l i t r e mul t i pl y by 0. 22mi l es per l i t re mi l es per gal l on ( UK) mul t i pl y by 4. 546mi l es per gal l on ( UK) ki l omet r es per l i t r e mul t i pl y by 0. 354




19/25

mi l es per gal l on ( US) mi l es per gal l on ( UK) mul t i pl y by 1. 2mi l es per gal l on ( US) mi l es per l i t r e mul t i pl y by 0. 2642mi l es per l i t re mi l es per gal l on ( US) mul t i pl y by 3. 785mi l es per gal l on ( US) ki l omet r es per l i t r e mul t i pl y by 0. 4251

Xmi l es per gal l on gal l ons per 100 mi l es : di vi de 100 by X( bot h gal l ons must of t he same t ype)

Xmi l es per gal l on ( UK) l i t r es per 100 km: di vi de 282. 5 by X

Xmi l es per gal l on ( US) l i t r es per 100 km: di vi de 235. 2 by XXkmper l i t r e l i t r es per 100 km: di vi de 100 by XXmi l es per l i t r e l i t r es per 100 km: di vi de 62. 14 by X


Power

Since power is a measure of the rate at which work is done, the underlying units are those ofwork or energy, andthat section should be looked at for explanations concerning the calorie and Btu . In this section the (IT) values havebeen used.In this section it is the horsepower which provides confusion. Just like the calorie, it can take 5 different values,

and these are identified as necessary by the addition of (boiler), (electric), (metric), (UK) and (water). Unlike thecalorie (whose 5 values are reasonably close to each other), the horsepower has 4 which are close and 1 (boiler)which is considerably different - it is about 13 times bigger than the others - but it seems to be very little used.

The S I unit of power is the watt. To change any of these other units of energy or work into their equivalentvalues in watts use the operation and conversion factor given. Those marked with # are exact. Other values aregiven to an appropriate degree of accuracy.

Bt u/ hour x 0. 293 071Bt u/ mi nut e x 17. 584 267Bt u/ second x 1055. 056cal or i es/ hour x 0. 001 639cal or i es/ mi nut e x 0. 069 78cal or i es/ second x 4. 1868 #f t l b- f or ce/ mi nut e x 0. 022 597

f t l b- f or ce/ second x 1. 355 82gi gawat ts [ GW] x 1 000 000 000horsepower (el ectr i c) x 746 #horsepower (met r i c) x 735. 499

wat t s [ W] 1

j oul es/ hour di vi de by 3600 #j oul es/ mi nut e di vi de by 60 #j oul es/ second 1ki l ocal or i es/ hour x 1. 163ki l ocal or i es/ mi nut e x 69. 78kg- f orce metr es/hour x 0. 002 724kg- f or ce met r es/ mi nut e x 0. 163 444

ki l owat t s [ kW] x 1000 #megawat t s [ MW] x 1 000 000 #


Pressure or Stress

The S I unit of pressure is the pascal. The units of pressure are defined in the same way as those for stress -force/unit area. To change any of these other units of pressure (or stress) into their equivalent values in pascalsuse the operation and conversion factor given. Those marked with # are exact. Other values are given to an




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appropriate degree of accuracy. Measures based on water assume a density of 1 kg/litre - a value which rarelymatched in the real world, though the error is small.

at mospher es x 101 325 #bar s x 100 000 #cent i met r es of mer cur y x 1333. 22cent i met res of wat er x 98. 066 5 #f eet of wat er x 2989. 066 92 #hect opascal s [ hPa] x 100 #i nches of wat er x 249. 088 91 #

i nches of mer cur y x 3386. 388kg- f or ce/ sq. cent i met re x 98 066. 5 #kg- f or ce/ sq. met r e x 9. 806 65 #ki l onewt on/ sq. met r e x 1000 #ki l opascal [ kPa] x 1000 #ki ps/ sq. i nch x 6 894 760meganewt ons/ sq. met r e x 1 000 000 #met res of wat er x 9806. 65 #mi l l i bar s x 100 #

pascal s [ Pa] 1

mi l l i met r es of mer cury x 133. 322mi l l i metr es of water x 9. 806 65 #

newt ons/ sq. cent i met re x 10 000newt ons/ sq. met r e 1newt ons/ sq. mi l l i met r e x 1 000 000 #pounds - f or ce/ sq. f oot x 47. 880pounds- f orce/ sq. i nch x 6894. 757poundal s/ sq. f oot x 1. 448 16t ons(UK) - f orce/ sq. f oot x 107 251t ons( UK) - f orce/ sq. i nch x 15 444 256t ons(US) - f orce/ sq. f oot x 95 760t ons( US) - f orce/ sq. i nch x 13 789 500tonnes - f or ce/ sq. cm x 98 066 500 #t onnes- f orce/ sq. met re x 9806. 65 #


Speed

The S I compatible unit of speed is metres/second . To change any of these other units of speed into theirequivalent values in metres/second use the operation and conversion factor given. Those marked with # areexact. Other values are given to an appropriate degree of accuracy.

cent i met r es /mi nute di vi de by 6000 #cent i met r es/ s econd di vi de by 100 #f eet / hour di vi de by 11 811f eet / mi nut e x 0. 005 08 #f eet / second x 0. 3048 #i nches/ mi nut e di vi de by 2362. 2

i nches/ second x 0. 0254 #ki l omet res/ hour di vi de by 3. 6 #ki l omet r es/ second x 1000 #knot s x 0. 514 444Mach number x 331. 5met r es/ hour di vi de by 3600 #met r es/ mi nut e di vi de by 60 #

metr es/ second [ m/ s] 1

mi l es/ hour x 0. 447 04 #




21/25

mi l es/ mi nut e x 26. 8224 #mi l es/ second x 1609. 344 #yar ds/ hour di vi de by 3937yar ds/ mi nut e x 0. 015 24 #yar ds/ second x 0. 9144 #


Spread Rate (by mass)

The spread rate of a substance is a measure of how much of it there is covering a unit area. The 'how much' can bemeasured by volume or by mass. The S I compatible unit of spread rate by mass is kilograms/square metre. It isalso a measure of area density (mass/unit area) and is similar to - but not the same as - pressure, which is force/unitarea. For the rainfall conversions a density of 1 kg/litre has been assumed. To change any of these other units ofspread rate into their equivalent values in kilograms/square metre use the operation and conversion factorgiven. Those marked with # are exact. Other values are given to an appropriate degree of accuracy. The conversionfor rainfall assumes a density of 1 kg/litre which is accurate enough for all practical purposes.

gr ams/ sq. cent i met r e x 10 #gr ams/ sq. met re di vi de by 1000 #i nches of r ai nf al l x 2. 54ki l ogr ams/ hect ar e di vi de by 10 000 #

ki l ogr ams/ sq. cent i metr e x 10 000 #mi l l i grams / sq. met r e di vi de by 1000 #mi l l i met r es of r ai nf al l 1

ki l ogr ams/ sq. metr e 1

ounces/ sq. f oot x 0. 305 152ounces/ sq. i nch x 43. 942ounces/ sq. yar d di vi de by 49. 494pounds/ acr e di vi de by 8921. 791pounds/ sq. f oot x 4. 882 428pounds/ sq. i nch x 703. 07pounds/ sq. yar d x 0. 542 492t onnes/ hect ar e di vi de by 10 #

t ons( UK) / acr e di vi de by 3. 982 942t ons( US) / acr e di vi de by 4. 460 896


Spread Rate (by volume)

The spread rate of a substance is a measure of how much of it there is covering a unit area. The 'how much' can bemeasured by volume or by mass. The S I compatible unit of spread rate by volume is cubic metres/squaremetre. However, this is a rather large unit for most purposes and so litres/square metre is often preferred. To changeany of these other units of spread rate into their equivalent values in litres/square metre use the operation andconversion factor given. Those marked with # are exact. Other values are given to an appropriate degree ofaccuracy.

cubi c f eet / acr e di vi de by 142. 913cubi c i nches /sq. yard di vi de by 51. 024cubi c yards /sq. mi l e di vi de by 3387. 577cubi c met res/ hectare di vi de by 10 #cubi c met r es/ sq. km di vi de by 1000 #cubi c met res/ sq. met re x 1000 #f l . ounces(UK) / sq. yard di vi de by 29. 428

l i t r es/ square metr e 1




22/25

gal l ons( UK) / acr e di vi de by 890. 184gal l ons( US) / acr e di vi de by 1069. 066gal l ons(UK) / hectare di vi de by 2199. 692gal l ons(US) / hectare di vi de by 2641. 721i nches of r ai nf al l x 25. 4 #l i t r es/ hect ar e di vi de by 10 000 #mi l l i l i t r es/ s q. met r e di vi de by 1000 #mi l l i met r es of r ai nf al l 1


Torque

The S I compatible unit of torque is the newton metre. To change any of these other units of torque into theirequivalent values in newton metres use the operation and conversion factor given. Those marked with # areexact. Other values are given to an appropriate degree of accuracy.

dyne cent i met r es di vi de by 10 000 000 #gr am- f or ce cent i metr es x 0. 000 098 066 5 #kg- f orce cent i metr es x 0. 098 066 5 #kg- f or ce met res x 9. 806 65 #

newton cent i met r es di vi de by 100 #

newt on met res [Nm] 1

ounce- f or ce i nches di vi de by 141. 612pound- f or ce i nches x 0. 112 984pound- f or ce f eet x 1. 355 818poundal f eet x 0. 042 140t on( UK) - f or ce f eet x 3 037. 032t on( US) - f or ce f eet x 2 711. 636t onne- f or ce met r es x 9 806. 65 #


Other Sources in Books




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Conversion Tables of Units for Science andEngineeringby Ari L HorvathMacmillan Reference Books, London, 1986 (147 pages)ISBN 0 333 40857 8Probably the most comprehensive set of conversion factorsin print, covering both old and modern units. There are 77tables covering categories from Length to Radiation dosage.

The Length table alone lists 107 units together with theconversion factors needed to change each one into metres.

The Dent Dictionary of Measurementby Darton and ClarkJ M Dent, London, 1994 (538 pages)ISBN 0 460 861379Very comprehensive coverage of all kinds of units(including currencies), ordered in conventional dictionaryform, and giving several conversion factors.

The Economist Desk CompanionRandom Century, London, 1992 (272 pages)ISBN 0 7126 9816 7

A handy compendium of units used in Science, Medicine,Engineering, Industry, Commerce, Finance and many otherplaces, together with all the necessary conversion factors.There is also much other incidental (but related)information.

The Encyclopaedia BritannicaThe modern E B has many references to units, but extensiveuse needs to be made of the index to find them all. It gives awide selection of weights and measures from countriesaround the world and the appropriate conversion factors.

World Weights and MeasuresStatistical Office of the United Nations, New York 1955(225 pages)A very comprehensive survey of each country in the world(as it was then) from Aden to Zanzibar, giving the unitsused in each for Length, Area and Capacity with theirBritish and Metric equivalents. There is an appendix on themeasures used for selected commodities. Currencies arealso given. The indexes are very thorough.

The Weights and Measures of Englandby R D ConnorH M S O, London, 1987 (422 pages)ISBN 0 460 86137 9A scholarly and detailed account of the history of development of the British (Imperial) system ofweights and measures from the earliest times.

British Weights and Measuresby R E ZupkoA history from Antiquity to the Seventeenth CentuThe University of Wisconsin Press, 1977 [248 paISBN 0 299 07340 8The actual history occupies only 100 pages. Therethen an extensive list of the various units used incommerce, tables of many pre-Imperial units, a lolist of pre-metric measures used in Europe togethewith their British and metric equivalents, and nearpages giving other sources.

The World of Measurementsby H Arthur Klein

Allen and Unwin, London, 1975 (736 pages)ISBN 0 04 500024 7A very readable and comprehensive account of thehistory of units used in measuring, from the earlieknown beginnings and around the world.

Scientific Unit Conversionby Francois CardarelliSpringer-Verlag, London, 1997 (456 pages)ISBN 3-540-76022-9It claims "This practical manual aims to be the mocomprehensive work on the subject of unit conveIt contains more than 10 000 precise conversionfactors."It is certainly a very chunky and compact (=handy-sized) book. Comprehensive it certainly is still not complete. However, with its very widecoverage, both historical and modern, it shouldcertainly satisfy nearly all users.

Other Sources on the World Wide Web

There are now several sites concerned with this topic.In the UK the two organisations concerned with standards are the

British Standards Institute (BSI) and the National Physical Laboratory (NPL).Sadly, the first offers nothing more than advertisements for its various books of standards, and the second is notvery useful on the subject of units and their conversion.Fortunately the Yahoo! site (based in the UK) does provide a gateway to many sources of information.

In the USA theNational Institute of Standards and Technology (NIST)is much more forthcoming, and there is no shortage of information concerning units and their conversion. There iseven an excellent 86-page book on the subject (SP 811) which can be read on-line or downloaded and printed out -but note thatAdobe Acrobat Readeris needed.The US Metric Association is also a good starting point which provides a wealth of links to other suitable sites.




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The International Standards Organisation] [I S O] based in Switzerland, is responsible for the world-widepublication of standards for just about anything for which standards can be set. Whilst none of the actual data isonline, details of the work of ISO and the publications they produce are. They also give many references to otherorganisations concerned with standards.

An excellent A to Z of units is available from this site run by Russ Rowlett at the University of North Carolina.

Another account of metrication and associated items which has, in addition, some very good pages on historic

measures (Anglo-Saxon, Biblical etc.) is provided by Jack Proot (in Canada)


Notes

ErrorsWhilst every care has been taken in thecompilation of this document, and manychecks have been carried out, the possibility ofan error is always present in a work like thisand that must be borne in mind by all users.The author would be glad to be told of anyerrors detected.

AccuracyIn a general dictionary like this it is impossibleto know just what accuracy is needed by anyparticular user. Where the given value is anexact one then it has been signalled. In mostcases other values are accurate to at least thenumber of significant figures shown. In somecases it might be more than that as trailingzeros have not been included.

PresentationThe conversion factors have mainly beenpresented as multipliers, but exceptions to that

have been made for two reasons. First, it iseasier to convey the exact value 'divide by 60'rather than the approximation 'multiply by0.0166667' and it is more likely to be keyed inwithout errors if a calculator is being used.Second, most calculators accept only 8 digits,which means that 'multiply by 0.000 084 666'will become '0.000 0846' (3 significantfigures) whereas 'divide by 11 811' will givethe result to 6 significant figures. Theappearance of a '1' needs no operator butshows that the named unit is exactly equivalentto the standard unit.

Inverse usageIn nearly all cases the conversion factors hbeen given to change 'non-standard' unitsstandard units of the SI. For those cases wit is necessary to do a conversion the otheit is only a matter of reversing the operatioFor example to convert feet into metres yomultiply by 0.3048 so, to convert metrefeet you divide by 0.3048. Following onfrom this it can be seen how conversions be made between non-standard units, chanfirst into the standard unit and then back inthe required unit.

Author's NoteA guiding principle behind the writing andpresentation of this document has been thclarity for non-specialist readers. To thatI have been guilty of breaking "the rules" few places. I am sorry that thesetransgressions may offend some readers b

have done so in the belief that it will be a lbit easier for many, and also help the flowcontinuous narrative.This dictionary is not meant to beencyclopaedic in its coverage, and there amany many more units which are not toucupon, but it is hoped that all 'ordinary' neare covered. The many references to othersources, both in books and on-line shouldcare of anything beyond that.Finally, I must thank all of those who wrowith suggestions (and corrections!) afterreading the earlier editions.

Queries, comments and (further) corrections will be welcomed by Frank Tapson


Go to C I M T Home Page.Publishing history19th June 1995 (First placed online)Updates:27th August 1997 (Minor corrections)




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21st November 1997 (Major corrections and alterations)20th January 1999 (Minor corrections and alterations)9th August 1999 (A few adjustments to links)13th December 1999 (Summary table of conversion factors added)


Diccionario de Unidades de Medida

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